1. Field of the Invention
The present invention relates to an artificial articulation for replacing an articulation portion of a living body.
2. Description of the Prior Art
The function of an articulation of a living body can be damaged in an accident, such as in injuries resulting from a traffic accident, and can also be damaged by diseases, such as rheumatism, which cause bone to deform. When the articulation is damaged extensively and the chance of recovery is small, a replacement operation is effected to remove the articulation, and an artificial articulation is mounted in its place.
Such an artificial articulation is required to provide the following characteristics:
(1) Since it is to be embedded in a living body for a long period of time, the artificial articulation must have an affinity with the surrounding bone and tissue;
(2) The artificial articulation must not deteriorate and degenerate over time, and various characteristics of the artificial articulation, such as mechanical strength, should remain constant;
(3) The pivotal portion of the artificial articulation must be durable and must be able to reproduce the basic articulate function of the true articulation which it replaces;
(4) The artificial articulation must be innoxious to the living body.
The mechanical strength of the bone insertion portion of the artificial articulation is a particularly important factor because force, which is several times as large as the weight of a living body, may be applied to portions of the artificial articulation due to muscular strength acting on it, in addition to the weight of the living body. Thus, a high mechanical strength is required for the artificial articulation.
In a conventional artificial articulation, as shown in FIG. 1, a stem 4, which is made of nickel-chrome alloy stainless steel or cobalt-chrome-molybdenum alloy stainless steel and which is to be inserted into a bone B1, is connected to the bone head ball member 3 which is made of alumina porcelain. A socket 1, which is made of high-density polyethylene, is fixed by bone cement 2 in a hip-bone B2, and the bone head ball member 3 is pivotally, slidably connected to the socket 1. However, because the bone head ball member 3 which is made of alumina porcelain pivotally slides in the socket 1 which is made of high-density polyethylene, the inner surface of the socket 1 is worn by about 0.2 mm a year and must be replaced every three years. Further, although the metal stem 4 is connected to the bone B1 with bone cement 5 in order to prevent the stem from directly contacting the bone B1, a chemical reaction may take place between the bone cement 5 and the metal stem 4 when they are embedded in the bone B1 for a long period of time, and this reaction often injures the living body. An alternative method of using calcium phosphate rather than bone cement to connect the metal stem 4 to the bone B.sub.1 has not proven effective because of insufficient bonding strength between the calcium phosphate and the metal stem.
In addition, though alumina porcelain does not react with bone cement, an alumina porcelain stem is not very stong and is particularly weak in deflective strength when compared with the metal stem so that there is a risk of breakage when large stresses are placed on an alumina porcelain stem during movement of the living body.